The present invention relates to a process of crystallizing a semiconductor thin film, a laser irradiation system used for carrying out the crystallization process, a thin film transistor fabricated by using the process and system, and a display using the thin film transistor.
Thin film transistors have been widely used as switching devices for liquid crystal displays and organic EL displays. In particular, a thin film transistor including an active layer made from poly-crystal silicon is advantageous in that not only switching devices and peripheral drive circuits can be provided on the same substrate. The poly-crystal silicon thin film transistor is also advantageous in that it can be made fine, to allow the opening ratio of the pixel structure to be increased. For these reasons, the poly-crystal silicon thin film transistor has received attention as a device for a high-definition display. In recent years, a technique of fabricating the poly-crystal silicon thin film transistor by using a low temperature process performed at 600xc2x0 C. or less has been actively studied. The adoption of the so-called low temperature process eliminates the need of using an expensive heat-resisting substrate, and thereby contributes to cost reduction and enlargement of the display. In particular, it has been increasingly required to pack not only switching devices for pixels and peripheral drive circuits but also a highly functional device represented by a central processing unit (CPU) on the same substrate. To meet such a requirement, it is expected to develop a technique of forming a poly-crystal silicon thin film having a high quality similar to that of a single crystal silicon thin film.
In accordance with the related art low temperature process, an excimer laser beam or electron beam formed in a long-sized shape or linear shape is scanned to irradiate the surface of a substrate on which amorphous silicon is previously deposited, to convert the amorphous silicon into poly-crystal silicon. Alternatively, the substrate is collectively irradiated with an excimer laser beam formed into a rectangular shape having a large area, to convert amorphous silicon into poly-crystal silicon. The irradiation of the substrate with a high energy beam such as a laser beam or electron beam can rapidly heat and melt only amorphous silicon on the substrate without giving damages to the substrate. The crystallization of silicon occurs at the subsequent cooling step, to result in an aggregation of poly-crystals having a relatively large grain size. For the energy beam having been used, however, the pulse continuation time is as very short as 20-200 ns. As a result, since a time required for amorphous silicon to be re-solidified after being melted is very short, so that the melted silicon is actually rapidly cooled and converted into poly-crystal silicon. The occurrence frequency of crystal nuclei becomes higher by rapid cooling of the melted silicon. As a result, the grain size of the poly-crystal silicon thus obtained becomes smaller. The mobility of the thin film transistor using the poly-crystal silicon having a small grain size as an active layer is as small as about 80 cm2/Vs for the N-channel type MOS transistor.
Accordingly, to pack a circuit having a high function, together with switching devices for pixels, on the same substrate, it is required to significantly improve the performance of thin film transistors. To meet such a requirement, there has been proposed a technique of irradiating a substrate with a laser beam in a state in which the substrate is heated at about 400xc2x0 C. By previously heating the substrate, the re-crystallization rate (cooling rate) after laser irradiation becomes slow, to thereby increase the crystal grain size. According to this technique, however, in the case of using a glass substrate, the upper limit of the heating temperature becomes about 450xc2x0 C. due to the thermal limitation of the glass substrate, which temperature is much lower than the melting point of silicon, that is, 1400xc2x0 C. As a result, even by adopting the method of pre-heating the substrate, poly-crystal silicon is rapidly cooled after laser irradiation, so that it is difficult to obtain poly-crystal silicon having a large crystal grain size similar to that of single crystal silicon.
Another method of forming poly-crystal silicon having a large crystal grain size is disclosed, for example, in Japanese Patent Laid-open No. Hei 7-297125, in which a catalyst metal is introduced in a silicon thin film for allowing crystals of silicon to grow in a specific crystal orientation. The method, however, basically requires a solid-growth process characterized in that annealing is performed at a temperature of 550xc2x0 C. or more, and therefore, the method is poor in matching with the low temperature process. Further, since a catalyst metal remains in the silicon thin film, it is required to provide the additional step of removing the metal component by gettering.
An object of the present invention is to provide a process of simply forming a silicon thin film having a crystallinity similar to that of single crystal silicon over a large area at a high throughput.
To achieve the above object, according to a first aspect of the present invention, there is provided a process of crystallizing a semiconductor thin film previously formed on a substrate by irradiating the semiconductor thin film with a laser beam, including:
a preparation step of dividing the surface of the substrate into a plurality of division regions, and shaping a laser beam to adjust an irradiation region of the laser beam such that one of the division regions is collectively irradiated with one shot of the laser beam;
a crystallization step of irradiating one of the division regions with the laser beam while optically modulating the intensity of the laser beam such that a cyclic light-and-dark pattern is projected on the irradiation region, and irradiating the same division region by at least one time after shifting the pattern such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting; and
a scanning step of shifting the irradiation region of the laser beam to the next division region, and repeating the crystallization step for the division region.
The crystallization step preferably includes a step of controlling the direction of crystallization by making use of a temperature gradient corresponding to the light-and-dark pattern, and irradiating the same division region again after shifting the pattern by a distance within a crystallization distance by one shot of laser irradiation. Further, the crystallization step is preferably carried out in a state in which the substrate is heated at a temperature of 200xc2x0 C. or more.
According to a second aspect of the present invention, there is provided a laser irradiation system for crystallizing a semiconductor thin film previously formed on a substrate by irradiating the semiconductor thin film with a laser beam, including:
shaping means for shaping, when the surface of the substrate is divided into a plurality of division regions, a laser beam to adjust an irradiation region of the laser beam such that one of the division regions is collectively irradiated with one shot of the laser beam;
optical means for optically modulating the intensity of the laser beam such that a cyclic light-and-dark pattern is projected on the irradiation region;
primary scanning means for irradiating one of the division regions with the optically modulated laser beam, and irradiating the same division region after shifting the pattern such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting; and
secondary scanning means for shifting the irradiation region of the laser beam to the next division region, and repeating the crystallization step for the division region.
The optical means preferably includes a micro-slit on which a cyclic light-and-dark pattern is depicted, the optical means being inserted in the optical path of the laser beam; or a phase shift mask which diffracts the laser beam to form a cyclic light-and-dark pattern, the optical means being inserted in the optical path of the laser beam.
According to a third aspect of the present invention, there is provided a thin film transistor including:
a semiconductor thin film;
a gate insulating film stacked on one surface of the semiconductor thin film; and
a gate electrode stacked on the semiconductor thin film via the gate insulating film;
the semiconductor thin film is obtained by forming amorphous silicon or poly-crystal silicon having a relatively small grain size on a substrate, and irradiating the amorphous silicon or poly-crystal silicon having a relatively small grain size with the laser beam to crystallize the amorphous silicon or poly-crystal silicon having a relatively small grain size into poly-crystal silicon having a relatively large grain size; and
the crystallization of the semiconductor thin film is performed by dividing the surface of the substrate into a plurality of division regions, and shaping a laser beam to adjust an irradiation region of the laser beam such that one of the division regions is collectively irradiated with one shot of the laser beam; irradiating one of the division regions with the laser beam while optically modulating the intensity of the laser beam such that a cyclic light-and-dark pattern is projected on the irradiation region, and irradiating the same division region by at least one time after shifting the pattern such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting; and shifting the irradiation region of the laser beam to the next division region, and repeating the crystallization for the division region.
According to a fourth aspect of the present invention, there is provided a display including:
a pair of substrates jointed to each other with a specific gap put therebetween; and
an electrooptic material held in the gap between the substrates;
a counter electrode formed on one of the substrates; and
pixel electrodes and thin film transistors for driving the pixel electrodes, which are formed on the other of the substrates, each of the thin film transistor including a semiconductor thin film and a gate electrode stacked on one surface of the semiconductor thin film via a gate insulating film;
wherein the semiconductor thin film is obtained by forming amorphous silicon or poly-crystal silicon having a relatively small grain size on the other substrate, and irradiating the amorphous silicon or poly-crystal silicon having a relatively small grain size with the laser beam to crystallize the amorphous silicon or poly-crystal silicon having a relatively small grain size into poly-crystal silicon having a relatively large grain size; and
the crystallization of the semiconductor thin film is performed by dividing the surface of the substrate into a plurality of division regions, and shaping a laser beam to adjust an irradiation region of the laser beam such that one of the division regions is collectively irradiated with one shot of the laser beam; irradiating one of the division regions with the laser beam while optically modulating the intensity of the laser beam such that a cyclic light-and-dark pattern is projected on the irradiation region, and irradiating the same division region by at least one time after shifting the pattern such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting; and shifting the irradiation region of the laser beam to the next division region, and repeating the crystallization for the division region.
According to a fifth aspect of the present invention, there is provided an organic EL display including:
pixel electrodes containing luminous portions made from an organic EL material and thin film transistors for driving the pixel electrodes, which are formed on a substrate, each of the thin film transistors including a semiconductor thin film and a gate electrode stacked on one surface of the semiconductor thin film via a gate insulating film;
wherein the semiconductor thin film is obtained by forming amorphous silicon or poly-crystal silicon having a relatively small grain size on the substrate, and irradiating the amorphous silicon or poly-crystal silicon having a relatively small grain size with the laser beam to crystallize the amorphous silicon or poly-crystal silicon having a relatively small grain size into poly-crystal silicon having a relatively large grain size; and
the crystallization of the semiconductor thin film is performed by dividing the surface of the substrate into a plurality of division regions, and shaping a laser beam to adjust an irradiation region of the laser beam such that one of the division regions is collectively irradiated with one shot of the laser beam; irradiating one of the division regions with the laser beam while optically modulating the intensity of the laser beam such that a cyclic light-and-dark pattern is projected on the irradiation region, and irradiating the same division region by at least one time after shifting the pattern such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting; and shifting the irradiation region of the laser beam to the next division region, and repeating the crystallization for the division region.
According to the present invention, the surface of a substrate is divided into a matrix pattern, to define a plurality of rectangular division regions, and correspondingly a laser beam is shaped into a rectangular shape such that one of the division regions can be collectively irradiated with one shot of the laser beam. The intensity of the laser beam is optically modulated such that a cyclic light-and-dark pattern is projected on the irradiation region shaped into the rectangular shape, and one of the division regions is irradiated with one shot of the laser beam. The direction of the crystallization is controlled by making use of a temperature gradient corresponding to the light-and-dark pattern. Then, the same division region is irradiated again with one shot of the laser beam after the pattern is shifted such that the light and dark portions of the pattern after shifting are not overlapped to those of the pattern before shifting. In this case, by performing the crystallization after shifting the pattern-by a distance within the crystallization distance by one shot of laser irradiation, it is possible to obtain more homogeneous crystals. The above crystallization step is repeated several times, and when the pattern is shifted by one cycle of the light or dark portions of the pattern, the irradiation region of the laser beam is shifted from the initial division region to the next division region, and the crystallization step is repeated for the division region. A large area semiconductor thin film can be thus effectively crystallized.